sit

A sit is localized subsidence or sagging of the coal seam roof, typically occurring in the worked-out or partially extracted areas of underground coal mines. Unlike a catastrophic fall, a sit develops gradually as the support structures deteriorate, the overlying strata loses cohesion, or mining extraction creates voids that the remaining roof cannot span. The term is regional to British mining, particularly in older coalfields.

Sits manifest as downward displacement of the immediate roof, often visible as cracking, buckling, or a visible bulge into the mine space. The extent varies widely: minor sits may only sag a few centimeters and affect a small area, while significant ones can drop the roof by half a meter or more across a working. The rate of movement matters operationally. A fast sit, developing over days or hours, poses immediate danger to workers and equipment below it; a slow sit, advancing over weeks, can sometimes be monitored and managed through supplementary propping or planned abandonment of the area.

Causes and Detection

Sits develop where the roof rock has low competence, poor bedding stability, or where water ingress has weakened the structure. Thick coal seams left partially unworked create unstable spans. Stone props become ineffective when they settle or the coal beneath them shifts, or when the immediate roof shales and mudstones lose strength under load and time. Detection relies on visual inspection, sounding the roof by striking it with a hammer to identify hollow or unstable sections, and monitoring for fresh cracks or dust falls from above.

The distinction between a sit and a fall hinges on mechanism and speed. A fall is sudden fracture and collapse of competent rock; a sit is creep-controlled subsidence of weakened or unsupported strata. In practice, a sit that goes unmanaged can develop into a fall. Modern ventilation requirements and ground control standards mean sits are now less tolerated operationally than they were in earlier mining eras, but the term persists in geological and safety documentation of older workings and heritage mines.